Silicon carbide has been a perennial candidate for use in semiconductor devices. Silicon carbide has long been recognized as having particular characteristics which give it excellent potential for producing semiconductor devices having superior characteristics to devices formed of other commonly used semiconductor materials such as silicon (Si), gallium arsenide (GaAs), and indium phosphide (InP). Silicon carbide has a wide bandgap, a high melting point, a low dielectric constant, a high breakdown field strength, a high thermal conductivity and a high saturated electron drift velocity. These characteristics give devices made from silicon carbide the potential to operate at higher temperatures, in closer proximity to one another, at higher power levels, and a number of other circumstances under which devices made from other semiconductor materials simply could not perform.
In spite of these known characteristics, commercial quality devices formed of silicon carbide have not been forthcoming. Silicon carbide is an extremely difficult material to work with which crystallizes in well over 150 different polytypes. Accordingly, forming the large single crystals of a single polytype or the thin films of particular polytypes of silicon carbide which are required to produce electronic devices on semiconductor materials has remained an elusive goal.
Recently, however, a number of developments have been accomplished in this field which have made the production of commercial quality electronic devices on silicon carbide possible for the first time. These developments are the subject of co-pending patent applications assigned to the common assignee of the present invention and include the following applications which are incorporated herein by reference:
Davis et al, "Growth of Beta-SiC Thin Films and Semiconductor Devices Fabricated Thereon," Ser. No. 07/113,921 filed Oct. 26, 1987; Davis et al.; "Homoepitaxial Growth of Alpha-SiC Thin Films and Semiconductor Devices Fabricated Thereon," Ser. No. 07/113,573, filed Oct. 26, 1987; Davis et al. "Sublimation of Silicon Carbide to Produce Large, Device Quality Single Crystals of Silicon Carbide," Ser. No. 07/113,565, filed Oct. 26, 1987; and Edmond et al "Implantation and Electrical Activation of Dopants into Monocrystalline Silicon Carbide," Ser. No. 07/113,561, filed Oct. 26, 1987, now abandoned.
As discussed in these applications, it is now possible to grow thin films of both alpha (6H hexagonal) and beta (3C cubic) thin films of silicon carbide where such thin films are required, and to appropriately grow large single crystals of silicon carbide where these are required.
One type of electronic device useful in a number of applications is the capacitor. One type of capacitor is known as a metal-insulator-semiconductor capacitor (MIS capacitor). Because silicon is the most commonly used semiconductor material and silicon dioxide is the most commonly used insulator material used in conjunction with silicon, these devices are most commonly referred to as metal-oxide-semiconductor capacitors (MOS capacitors). The term MOS capacitor will be used throughout this application, but it will be understood that the discussions are appropriately applied to MIS capacitors as well.
MOS capacitors can be used, for example, as temperature sensors, by monitoring the shift in threshold or flatband voltage of the capacitor with changes in temperature. Additionally, at a given temperature, a MOS capacitor can be used as a gas sensor by monitoring the shift in the threshold or flatband voltage as a function of the partial pressure of the gas. Additionally, as stated earlier, MOS capacitors could be used as capacitors in various circuit designs, particularly in conjunction with other devices formed from silicon carbide.
Several researchers have described attempts to produce MOS capacitors on silicon carbide. Suzuki et al., Thermal Oxidation of SiC and Electrical Properties of Al-SiO.sub.2 -SiC MOS Structure, Jap. J. Appl. Phys., Vol. 21, No. 4, p. 579, 1982, discuss MOS structures having an aluminum, silicon dioxide and silicon carbide structure, but in which ohmic contacts were made to the back side of the semiconductor substrate, i.e. the side opposite the metal-semiconductor contact. This particular positioning of the contact leads to a certain amount of series resistance in the device which in turn limits its capacitance range. Such series resistance is not typically a problem for MOS capacitors formed on silicon. Silicon carbide, however, is a much more resistive material, and exhibits decreased capacitance in MOS capacitors formed according to Suzuki's design.
Shibahara et al., Metal-Oxide-Semiconductor Characteristics of Chemical Vapor Deposited Cubic-SiC, Jap. J. Appl. Phys., Vol. 23, No. 11, 1984, p. L-862, also discuss an aluminum and silicon dioxide MOS capacitor on a 3C silicon carbide thin film. Shibahara, however, made the ohmic contact on the back of a 10 micrometer thin film of silicon carbide after a silicon substrate had been etched off. Such a device design would be far too thin and difficult to handle for any practical commercial applications.
Two other researchers, Fung et al., Thermal Oxidation of 3C Silicon Carbide Single Crystal Layers on Silicon, Appl. Phys. Lett., 45(7), Oct. 1984, p. 757; and Avila et al., Behavior of Inversion Layers in 3C Silicon Carbide, Appl. Phys. Lett., 49(6), Aug. 1986, p. 334, also studied MOS structures with double-column mercury probes. These discussions are characterization techniques, however, and do not represent viable device designs.
Accordingly, there exists no suitable device design for producing commercial quality MOS capacitors on silicon carbide.
Therefore, it is an object of the present invention to provide a metal-insulator-semiconductor capacitor with reduced series resistance and lesser leakage current, suitable for operation at high temperatures and high radiation densities. The capacitor comprises a semiconductor portion, an active portion of insulated material upon the semiconductor portion, a metal portion upon the insulator portion for defining the active region, and an ohmic contact upon the semiconductor portion and positioned in close proximity to both the active portion of the insulator and the active region of the semiconductor.
It is another object of the invention to provide an MOS capacitor wherein the semiconductor portion is formed of monocrystalline silicon carbide.
It is a further object of the invention to provide an MOS capacitor wherein the ohmic contact on the semiconductor portion is positioned in direct contact with the insulator portion.